The demand for non-aqueous secondary batteries such as a lithium-ion battery is increasing year after year due to a need for high voltage and high energy density. However, while the energy density of a battery is increased, the safety thereof is decreased. Therefore, the enhancement of safety is important in batteries with high energy density. Furthermore, implementing typical safety measures, the energy density tends to decrease. Therefore, there is a demand for measures in which safety is improved while energy density is maintained.
Conventionally, a special additive is contained in an electrolyte solution of a non-aqueous secondary battery to ensure overcharging safety. As an additive to an electrolyte solution, which is polymerized under a high voltage to improve safety during overcharging, biphenyl described in JP 9(1997)-171840 A, cyclohexylbenzene described in JP 2001-015155 A, and the like have been proposed. These additives generate gas during overcharging to facilitate the operation of a current interrupting valve. Thus, these additives ensure safety in combination with a current interrupting valve. However, a prismatic battery is not usually provided with a current interrupting valve, so that the effect of an additive cannot be exhibited sufficiently. Furthermore, when a large amount of an additive to the electrolyte solution is used, particularly, in combination with a positive electrode having a large surface area, the stability of the additive in a charged state is decreased. When this state is left at a high temperature for a long period of time, the electrolyte solution is decomposed and the like, which is likely to cause a battery to expand and an internal resistance to increase. Due to the gas generated at this time, the internal pressure is increased in the case of a prismatic battery, and battery expansion occurs in the case of a prismatic battery and a laminated battery. Particularly, in the prismatic battery and the laminated battery, the apparent size is changed.
On the other hand, when fluorobenzene or the like is used as an additive to the electrolyte solution, a battery is unlikely to expand and the like, although the addition of a small amount of fluorobenzene or the like has a small effect on overcharging since fluorobenzene itself is stable under a high voltage. For example, when the added amount of fluorobenzene is increased, overcharging performance becomes better; however, the effect of the increased added amount is not so high as that of cyclohexylbenzene.